Millicent Bullstrode, identified by last name only ("Miss Bullstrode"), is partnered with Hermione Granger in the Dueling Club. While we don't see exactly what happens in their duel, the end result is that Millicent has Hermione in a headlock, which Harry ends up breaking. Hermione later tries to use a hair that Millicent left on her in a Polyjuice potion to turn into Millicent and sneak into the Slytherin common room; unfortunately this fails as the hair is not in fact one of Millicent's own.

Millicent is one of the Slytherin students who are appointed to the Inquisitorial Squad by Professor Umbridge.

Millicent pins Hermione to the wall of Umbridge's office after Harry and the others are captured while trying to contact Sirius Black. We are led to believe that it was Millicent who captured Hermione, although we can't be sure because this action happens off stage.